The twin Voyager probes are the ultimate spaceflight overachievers, but everyone knows their run can't last forever.Right now, it's looking like the grizzled spacefarers have about five years before they fall silent, when they'll be no longer able to send word of their adventures back to the humans who have eagerly awaited their telegrams for 42 years and counting. The Voyagers' journey will continue indefinitely, but we will no longer travel with them."It's cooling off, the spacecraft is getting colder all the time and the power is dropping," Ed Stone, the mission's project scientist and a physicist at Caltech, said during a news conference held Oct. 31 in conjunction with the publication of a handful of new scientific papers. "We know that somehow, in another five years or so, we may not have enough power to have any scientific instruments on any longer."